1. Field of the Invention:
This invention relates to electrolytic drainage treating apparatus.
2. Description of the Prior Art:
It is known to provide electrolytic treatments for cleaning drainages by removing contaminated components in suspension or solution with the flocculating effect of aluminum ions which are eluted by the electrolysis of aluminum. This has been found useful in cleaning drainages containing an insoluble colloidal suspension of ions of organic compounds or inorganic compounds such as dyeing drainages, food industrial drainages, and acidic drainages.
Referring to FIG. 1, the structure and operation of a typical conventional electrolytic treating apparatus will be illustrated. In FIG. 1, reference numeral 1 designates a drainage fed to an electrolyzer 9; 3 designates an anode made of aluminum; and 4 designates a cathode made of stainless steel or mild steel. A plurality of anode plates and cathode plates are connected in parallel to which DC current is supplied from a DC power source 6, 5 designates an outlet of the treated drainage.
In the apparatus shown in FIG. 1, aluminum of anode 3 is eluted as ions by passing DC current across the plates whereby the contaminated components are flocculated by the effect of the aluminum ions to form floc. The floc is floated with gas generated by the electrolysis (mainly hydrogen gas generated from the cathode) becoming scum 8 which is floated on the surface of the electrolytic solution in the electrolyzer 9. The scum 8 is scraped off by a skimmer 7 and clean water is discharged from the bottom of the electrolyzer.
However, in the above-mentioned electrolytic treating apparatus, the equivalent resistancee between the electrodes is increased which prevents current flow and which prevents electrolytic operation by contamination of the electrode surface caused by surface coated material (hereinafter referring to as scale) and by deposition of gas and other contaminated materials.
Because of the scale, the electrodes are rendered inactive. The inactivated state is caused after tens of hours or may occur after hundreds of hours of continuous operation. The surface electric current density of the electrode in the above noted operation is usually low such as about several mA/cm.sup.2. Because of this, large surface electrodes are used and a plurality of electrode plates are arranged in parallel with gaps of several cm. The current density of the electrodes is kept low in order to decrease the rate of elute per unit area of the anode so as to maintain a relatively long term active operation of the electrodes. Several tens of volts have been found necessary for the electrode gaps of several cm. For example, the treatment of dyeing drainage at a rate of 20m.sup.3 /hour requires 50 ppm of aluminum ions. The current required for the electrolysis is about 3000 A. The electrode surface S is given by the equation ##EQU1## wherein current density J = 2mA/cm.sup.2.
When an electrode plate having an area of 1 m.sup.2 is used and current is passed from both surfaces of the electrode, about 75 sheets of anode plates are required with the result that about 150 sheets of anode and cathode plates are required. Accordingly, the largeness of the electrolyzer required creates quite a burden.
It is usual to decrease the specific resistance P of the drainage by adding electrolyte. The specific resistance P can be about 500 ohms. The ohmic drop V between the electrodes is given by the equation
V = JPg = 2 .times. 10.sup..sup.-3 .times. 500 .times. 5 = 5 volts wherein the gap between electrodes electrodes g is 5 cm.
The equivalent resistance in the gap is quite higher than P because of the effects of scale and polarization. In practice, about 30 - 40 volts of applied voltage is necessary. In order to generate current in the case of an increase of electrode equivalent resistance caused by an increase of scale, it is usual to provide a DC power source of 50 - 100 volt which is greater than the ohmic drop.
However, this is not economical because of the requirements of a large power source. Thus the passive state caused by the scale etc. cannot be prevented by this procedure because of its impracticality.
A dyeing drainage contains a large quantity of colloid with the result that the colloid is flocculated with aluminum ions by the electrochemical method to obtain a clean drainage. However, in the conventional process, the separation of the scale by self-diffusion is not possible because the velocity of the electrolytic solution in the gap is too slow whereby the polarization on the surface of the electrodes is high. The drainage having high specific resistance is used as the electrolytic solution. Accordingly, it is difficult to perform the electrolysis in high current concentration and density and the electrolysis having low current concentraion of about several hundreds mA/1 and low current density of several hundreds mA/cm.sup.2 has been performed. Therefore, in order to give the coulometric concentration for eluting aluminum ions required for the cleaning of the drainage, it is necessary to electrolyze for an extended period. This has proved to be impractical for large volume drainage.
Moreover, in the conventional process, the scum flocculated with the aluminum ions eluted from the anode is fine floc. The scum is floated moving gradually in the gap from the anode. The floated scum 8 is discharged from the electrolyzer 9 by a skimmer 7. The gas used for the float of the floc is hydrogen gas which is generated on the cathode surface. The gap between the electrodes is quite broad, i.e., 10 - 100 mm. The diffusion of hydrogen gas is not sufficient and the fine bubbles of hydrogen gas are combined to form large bubbles which float at high velocity whereby the electrolytic solution is stirred to decrease the effect of flotation of the floc. The separation of the floc by floating is performed in the electrolyzer 9 between the electrodes at the time of electrolysis. Accordingly, the time for forming the floc from the insoluble suspension is too short. The direction of flow of the electrolytic solution of the drainage in the gap between the electrodes is from the upper part to the lower part as shown in FIG. 1. The fine floc is mixed in the treated drainage 5 making it difficult to obtain clean treated drainage. These are some of the disadvantages which have been found in the conventional apparatus.